This invention relates to the measurement of the dielectric constant of a fluid mixture and specifically the measurement of the amount of water contained in an oil/water mixture.
It is very useful to know the water content of an oil/water mixture for a number of applications in the petroleum industry. Measuring the water content of the oil emerging from an oil well can give an indication of the general health of the well and whether or not the well is being pumped too fast. Measuring the water content of oil emerging from a treating facility provides feedback on the efficiency of the treating operation. And, measuring the water content of oil at the point of custody transfer assures the buyer and seller of the quantities of oil and water being transferred.
The prior art contains a number of instruments for producing such a measurement. A number of these devices rely on somehow measuring the dielectric constant of the oil/water mixture. Because the relative dielectric constant of oil is in the range of 2 to 3 and that of water is approximately 80 the presence of water in the oil has a significant effect on the dielectric constant of the mixture.
One such method for measuring the dielectric constant is the capacitance method. Devices covered by U.S. Pat. No. 3,200,312 to Callahan, U.S. Pat. No. 3,025,464 to Bond, U.S. Pat. No. 3,523,245 to Love et al, U.S. Pat. No. 5,929,342 to Thompson, and U.S. Pat. No. 4,774,680 to Agar are examples of this method applied to the measurement of water in oil/water mixtures. Other applications to which the capacitance method is applied to determine water content can be seen in U.S. Pat. No. 4,769,593 to Reed et al, U.S. Pat. No. 4,864,850 to Price, and U.S. Pat. No. 4,559,493 to Goldberg et al.
Another class of methods for determining the water content involves the use of microwaves. There are a few different ways that microwaves can be used.
U.S. Pat. No. 5,103,181 to Gaisford et al describes a device in which the oil/water mixture is made to flow through a section of pipe that has been modified to look like a resonant cavity. Microwave energy is introduced into the container and forms constructive and destructive interference at various positions within the cavity. Two microwave detectors are positioned at the side of the container at predetermined positions. The frequency of the microwave energy is adjusted until the phase of the signal at each receiver is exactly in phase or out of phase. The frequency at which this occurs is used to deduce the dielectric constant of the fluid within the resonant cavity.
U.S. Pat. No. 5,101,163 to Agar describes a form of microwave sensor similar to Gaisford et al. Agar places the microwave transmitter and two microwave receivers within a pipe containing the fluid to be measured. The pipe acts as a microwave waveguide instead of a resonant cavity. The receivers are positioned on either side of the transmitter at specified distances from the transmitter. The transmitter emits microwave energy into the waveguide. This energy is received by the receivers. The frequency of the microwave energy is adjusted until the signals at the receivers are in phase or out of phase. The frequency at which this occurs is used to deduce the dielectric constant of the fluid within the pipe.
U.S. Pat. Nos. 4,862,060; 4,996,490; 5,025,222; 5,157,339; and 5,748,002 to Scott et al describe the use of microwave load pulling to determine the dielectric constant of the oil/water mixture. The fluid to be tested flows through a resonant cavity. A microwave oscillator is electrically coupled to the resonant cavity. The dielectric constant of the fluid in the cavity alters the resonant characteristics of the cavity. The change in the frequency and amplitude of the microwave oscillations is used to deduce the dielectric constant of the fluid within the resonant cavity.
U.S. Pat. No. 5,926,024 to Blount et al describes a system similar to that of Scott et al that has been modified to work in down-hole applications.
U.S. Pat. No. 5,351,521 to Cracknell describes another method of using microwaves to deduce the dielectric constant of an oil/water mixture within a pipe. One or more pipe sections, each having a diameter smaller than the pipe containing the mixture, are inserted into the pipe. Microwave energy is transmitted along the pipe, through the smaller pipe sections, to a receiver. The pipe and pipe sections act as a waveguide. Thus, this geometry has an upper limit to the wavelength it will allow to propagate. The frequency of the microwave energy is lowered, increasing the wavelength until the receiver stops receiving the energy. This cutoff frequency is related to the geometry of the pipe and pipe sections and the dielectric constant of the fluid filling the pipe.
Another method of determining the water content involves the use of time domain reflectometry or TDF Generally, a TDR instrument consists of a signal source capable of supplying a voltage step with a very short rise time or voltage pulse with very short transition times, a transmission line, a probe that somehow interacts with the physical variable to be measured, and a timing circuit. The operation of the instrument consists of generating the signal propagating the signal along the transmission line to the probe, having the probe reflect the signal in some fashion, propagating the reflected signal back to the signal source, and measuring the interval of time between the generation of the signal and the return of the reflected signal. The probe can be fashioned to interact with one or more physical parameters. Two of these parameters are dielectric constant and fluid level.
If a known length of the probe is immersed in the material to be measured, the material not necessarily being an oil/water mixture, the measured time interval can be used to deduce the propagation velocity of the signal along the portion of the probe immersed in the material. This propagation velocity can, in turn, be used to deduce the dielectric constant of the material surrounding the probe. Examples of this kind of sensor are given in U.S. Pat. No. 3,965,416 to Friedman, U.S. Pat. No. 3,995,212 to Ross, U.S. Pat. No. 4,786,857 to Mohr et al, U.S. Pat. No. 5,459,403 to Kohler et al, U.S. Pat. No. 5,554,936 to Mohr, U.S. Pat. No. 5,723,979 to Mohr, U.S. Pat. No. 5,729,123 to Jandrasits et al, U.S. Pat. No. 5,818,241 to Kelly, and U.S. Pat. No. 5,898,308 to Champion.
U.S. Pat. No. 4,429,273 to Mazzagatti describes a probe geometry for an oil/water monitor. Mazzagatti briefly mentions two kinds of excitations that may be used with this geometry, but does not reveal the details of the monitoring means.
According to an aspect of the invention there is provided a method of determining a property of a fluid, such as water content of a hydrocarbon stream, that involves the use of time domain transmissometry or TDT. In TDT, the signal of interest enters the probe or sensor via one port, interacts with the fluid to be measured as it travels along the probe, and exits the probe via another port.
TDT has two main advantages over TDR. First, the signal only travels along the probe once in TDT whereas it makes two transits of the probe in TDR In cases where the bulk electrical conductivity of the fluid to be measured is high, the propagation losses of the signal while it travels along the probe will be high. In the case of TDR the signal must propagate a distance that is twice that of TDT so the losses will be at least a factor of 2 larger. Second, the receiver for a TDR instrument must distinguish the reflection of interest from a variety of other reflections caused by the system. When the amplitude of the reflected signal of interest is low this distinction can be difficult, even for a trained human observer. In the case of TDT, the first signal reaching the receiver is the signal of interest. Any subsequent reflected signals reaching the receiver are much less of a concern.
Therefore, there is provided a sensor for measuring the dielectric constant of a fluid, the sensor comprising:
a conduit for the fluid, the conduit having first and second ends, between which electrical energy may pass;
an electrical generator having as output an electrical transient, the electrical generator being operably connected to the first end of the conduit for transmitting the electrical transient along the conduit, wherein propagation of the electrical conduit is affected by the fluid;
a receiver connected to the second end of the conduit for detecting electrical transients that have passed along the conduit from the electrical generator; and
a processor operably connected to the electrical generator and to the receiver, the processor being programmed to cause an electrical transient to be generated by the electrical generator for passage along the conduit and determine the dielectric constant of the fluid from characteristics of the electrical transient detected at the receiver.
According to a further aspect of the invention, the conduit is a transmission line, the transmission line having a dielectric arranged so that the dielectric of the transmission line includes the fluid to be measured.
According to a further aspect of the invention, the conduit is preferably enclosed within a pressure container.
According to a further aspect of the invention, a characteristic of the electrical transient used by the processor is the time interval between the generation of the electrical transient by the electrical generator and the reception of the electrical transient at the receiver.
According to a further aspect of the invention, the electrical generator generates a further electrical transient upon receipt of an electrical transient at the receiver, thereby forming a sing-around circuit, and the sensor further comprises a timing circuit for measuring the oscillation period of the sing-around circuit, the timing circuit being configured to output a timing signal to the processor.
According to a further aspect of the invention, the sensor is provided with a reference circuit that has similar electrical delay to the sensor components, but is not affected by the fluid to be measured. In this aspect, the processor is configured to determine the dielectric constant of the fluid taking into account delay of the electrical transient due to passage through electrical components that are common to both the sensor and the reference circuit.
According to a further aspect of the invention, the transmission line is formed from co-axial inner and outer conductors, and has an annulus between the inner and outer conductors, with fluid to be measured filling the annulus.
According to a further aspect of the invention, the outer conductor forms a wall of the container. According to a further aspect of the invention, the transmission line comprises first and second parallel side-by-side conductors separated by a gap, the gap being substantially filled by the fluid to be measured.
According to a further aspect of the invention, the one or both of the conductors is coated with one or more layers of electrical insulation, and preferably the electrical insulation extends a distance into the annulus (or across the gap) sufficient to exclude fluid being measured from areas of greatest electric field intensity. A top layer should be selected for its insulation capabilities, while lower layers may be selected for ease of providing a thick layer.
According to a further aspect of the invention, the receiver, and also the second receiver in the core, comprises a comparator for comparing the voltage level of signals received by the receiver with an adjustable threshold, the time of arrival of the electrical transient being the time at which the voltage level of signals received by the receiver exceeds the threshold.
According to a further aspect of the invention, the processor is programmed to:
A) adjust the adjustable threshold to a value that is below the maximum voltage level of the electrical transient received by the receiver;
B) determine the travel time of the electrical transient along the transmission line for the adjusted value of the adjustable threshold;
C) repeat steps A and B for different values of the adjustable threshold to form a set of data pairs comprising the adjustable threshold and the travel time for each setting of the adjustable threshold; and
D) determine the dielectric constant and the bulk conductivity of the fluid being measured from the data pairs.
According to a further aspect of the invention, the processor is programmed to find a curve that matches the data pairs, determine the bulk conductivity of the fluid being measured from an extrapolation of the curve at a travel time corresponding to the base line of the electrical transient and correct the time interval using the determined bulk conductivity of the fluid.
According to a further aspect of the invention, there is provided a sensor having improved selectivity of transmission modes along the transmission line. According to this aspect of the invention, there is provided a sensor for determining a property of a fluid, the sensor comprising:
a transmission line extending between a first end and a second end, the transmission line being defined by co-axial inner and outer conductors, the inner and outer conductors being spaced apart to define a gap through which gap a fluid may flow;
an electrical generator connected through a distributed connection to the transmission line for transmitting electromagnetic energy into the transmission line;
a receiver operably connected to the transmission line for receiving electromagnetic energy that has passed along the transmission line from the electrical generator; and
a processor connected by first and second communication links respectively to the electrical generator and the receiver for processing signals received by the receiver and for generating a signal indicative of a property of fluid confined within the gap from signals received by the receiver that have passed along the transmission line.
According to a further aspect of the invention, the property is the dielectric constant of the fluid, and the processor is programmed to calculate the dielectric constant of the fluid from the time of flight of electrical signals along the transmission line.
According to a further aspect of the invention, the distributed connection comprises first and second feed lines, preferably in the form a co-axial cables, the first feed line being electrically connected to the inner conductor, and the second feed line being electrically connected to the outer conductor, and the second feed line being electrically connected to the outer conductor at multiple positions around the outer conductor.
According to a further aspect of the invention, the second feed line is electrically connected to the outer conductor at least at three positions.
According to a further aspect of the invention, the connection positions are spaced uniformly around the outer conductor.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.